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# Why in the world would Einstein suggest... 2

Olivier5

0
Sat 16 May, 2015 07:29 am
@Olivier5,
Guess there're quite a few loose ends i need to tie...

o5 wrote:
If Michelson and Morlet had used a Sagnac interferometer (one where two rays of light turn around some space in opposite directions) rather than a cross-shaped interferometer, they would have found their "aether"... Or believed they did. They would have seen the sort of shift in the interferences that they were looking for. They would have proven the earth movement with lights and mirrors... But a Sagnac interferometer can only detect rotation, not inertial translation.

Does that mean that rotation is absolute, while translation is not?

(TBC)

First, let see what the earth's rotation is relative to: the sun, the stars, etc. We can agree that either the stars turn around an immobile earth, or our earth itself turns around an axis as a toupee, generating the illusion of a rotating universe. Whatever the case, the rotation is relative to something considered (at least in first approximation) as immobile.

Lay asked at some point why we couldn't consider earth as immobile and the sky turning around it, as was the view before Copernicus. The answer, paraphrasing PoincarĂ©, is that we could, but the laws of sideral motion would be immensely more complicated than in our current non-earth centric system, and much harder to understand intuitively. Galaxies far away will go so very fast, and all that... It wouldn't be elegant nor convenient, but it can be done.

In SR terms, a summary of the above would be: The laws of motion and light and time, as SR expresses them, work when measured through any inertial frame, but get APPARENTLY messed up when the universe is observed using a frame that's affected by non-inertial movements, such as rotation or acceleration. Like the balls on a billiard table riding on a roller-coaster would get all messed up, a frame's own acceleration (in the broad physical sense of the word ie including rotation, vibration, change in direction, etc) introduces apparent perturbations and oddities when reading and plotting movements in that frame.

So in summary, the earth rotates around its north south poles axis when observed in any inertial frame. Any such frame would allow one to plot the movement of the entire solar system the same way, and would yield the exact same results in terms of orbits, sideral years, precessions, etc.

Does that mean the rotation of the earth is absolute? It depends...

If the pre-copernician situation described above does not bother you, you agree that earth's rotation is relative.

What if it DOES bother you? What if you'd rather think you're not the center of the universe to benefit from much simpler physics? Are you then entitled to conclude that earth's rotation is absolute?

I guess yes, if your definition of absolute is that which, if not true, would result in absurdly and needlessly complicated physics. My definition of "absolute" is different: I think it means "in and by itself, not relative to anything else". Under this definition, the earth doesn't really turn "in and by itself". It turns relative to the rest of the universe.
layman

2
Sat 16 May, 2015 07:51 am
@Olivier5,
Quote:
My definition of "absolute" is different: I think it means "in and by itself, not relative to anything else".

Your definition is yours. That's not the "scientific" definition. As I have told you before, in SR (or just general) terms, "absolute" just means "not frame dependent," whereas as "relative" just means "frame dependent."

That said, "absolute," even in SR, does basically imply that absolute motion is, using your words, "in and by itself, not relative to anything else." It is not a matter of which frame you're in when viewing it. It will be "seen" as such from all frames.

In terms of relative vs absolute simultaneity the difference is as follows:

Absolute simultaneity says that time dilation is, as the paper I just cited put it, "directional," i.e., it goes only one way, not two or more. It is not "frame dependent."

Relative simultaneity basically says that simultaneity (and therefore time dilation) is frame dependent. It says that time dilation, etc., is non-directional, i.e., "reciprocal." When expressed in everyday terms, it says that each clock "really is" slower than the other (when, but only when, two clocks are travelling "inertially", but at different relative speeds).

Olivier5

0
Sat 16 May, 2015 09:03 am
@layman,
Quote:
Your definition is yours. That's not the "scientific" definition. As I have told you before, in SR (or just general) terms, "absolute" just means "not frame dependent," whereas as "relative" just means "frame dependent."

Correct, and therefore earth's rotation is relative TO ANY INERTIAL FRAME. In an earth-bound frame of reference, the earth doesn't move.
0 Replies

layman

1
Sat 16 May, 2015 10:04 am
@Olivier5,
Quote:
the earth rotates around its north south poles axis when observed in any inertial frame. Any such frame would allow one to plot the movement of the entire solar system the same way...

No, not using SR anyway. Take an intertial frame traveling at .99999 c relative to earth, for example. All distances and times (of the sidereal years, etc.) would be different. As would supposed "simultaneity" (i.e. what happens when? which happens first, and which second, etc).

Now take one traveling at .98c. Yet another set of times, distances, etc. Now take one at .97c, ad infinitum. You would get an infinite number of different answers using those frames (all inertial frames) as the basis for calculations. Would all of those be "the same way?" Would ALL of those be right?

If you want to say that you could use those frames to "translate" (using the LT) back to how the earth would see it, I would agree. But doing that would VIOLATE the mandates of SR. You could, of course, do it using LR, though. Of course, calculating "how the earth would see it" using SR, would simply mean that everything else in the solar system (or the entire universe for that matter) would be moving IF it was moving relative to the (posited by SR to be) "stationary" earth. And so, that too would give you the answer you are trying to avoid.
Olivier5

0
Sat 16 May, 2015 10:32 am
@layman,
Quote:
If you want to say that you could use those frames to "translate" (using the LT) back to how the earth would see it, I would agree. But doing that would VIOLATE the mandates of SR.

No it wouldn't. It's perfectly doable in SR. What "mandates" are you talking about?

Quote:
Of course, calculating "how the earth would see it" using SR, would simply mean that everything else in the solar system (or the entire universe for that matter) would be moving IF it was moving relative to the (posited by SR to be) "stationary" earth. And so, that too would give you the answer you are trying to avoid.

??? Could you be more confused?
layman

1
Sat 16 May, 2015 11:02 am
@Olivier5,
Quote:
No it wouldn't. It's perfectly doable in SR. What "mandates" are you talking about?

You can "do the math" using SR, precisely because SR steals the LT from an absolute simultaneity theory. But, in SR, it would be a PROHIBITED mathematical calculation, theory-wise.
Olivier5

0
Sat 16 May, 2015 11:14 am
@layman,
No. That's precisely what the Lorentz transformations are for.
layman

1
Sat 16 May, 2015 11:19 am
@Olivier5,
Quote:
No. That's precisely what the Lorentz transformations are for.

What the LT are "for" depends on the theory you subscribe to, Ollie.

A point you continue to fail to grasp despite 100's of attempts to explain it to you.
Olivier5

0
Sat 16 May, 2015 04:49 pm
@layman,
A point which is quite simply false.
0 Replies

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